1,Alpha,25-dihydroxy vitamin D (1,25-(OH)2D) is a steroid hormone which regulates transepithelial calcium transport in its target organs, intestine and bone. The structural and cellular heterogeneity of bone has hampered the investigation of 1,25-(OH)2D action in calcified tissues. However, we have identified 1,25-(OH)2D receptors in two tissues which are potential model systems for bone, the chicken oviduct shell gland and the chick embryo chorioallantoic membrane (CAM). These specialized Ca++-transporting epithelia carry out mineral deposition and resorption, and large amounts of tissue can be obtained for studies of 1,25-(OH)2D action at the biochemical level. The objectives of the proposed research are to study the effects of 1,25-(OH)2D on the shell gland and CAM, and to correlate these effects with normal variations in tissue Ca++ transport during the egg-laying cycle and embryonic development as well as the binding of 1,25-(OH)2D to its receptors. Specifically, we will investigate the effects of 1,25-(OH)2D administration on shell gland total protein synthesis and labeling patterns, Ca++-dependent ATPase, calcium-binding protein and shell matrix proteoglycan biosynthesis. We will also examine the effect of 1,25-(OH)2D on CAM protein synthesis and labeling as well as Ca++ transport, calcium-binding protein and neuraminidase activity. The long range goal of these studies is to establish whether the shell gland and CAM are target organs for the steroid hormone 1,25-(OH)2D, and to determine the biochemical mechanism and hormonal regulation of the transepithelial calcium transport processes in these tissues. Since these organs perform functions associated with bone, results obtained in the proposed studies should prove useful in understanding the regulation of bone mineral metabolism by 1,25-(OH)2D. In addition, the shell gland is a reproductive organ whose growth and differentiation are regulated by estrogens and other steroids. Our preliminary evidence suggests that estrogens regulate shell gland 1,25-(OH)2D receptor levels. We will determine the dependence of estrogen induction of 1,25-(OH)2D receptor on DNA, RNA and protein synthesis, as well as the effects of other steroids (progestins, androgens and glucocorticoids) involved in bone deposition and shell formation. These studies may relate to reproductive hormone-linked bone disease, such as post-menopausal osteoporosis.